High-power RF cavities, such as those found in an RF LINAC, require not only tremendous RF powers (on the order to 10's to 100's of kW and above), but also a vacuum environment to prevent arcing and sparking within the RF cavity due to the intense electric fields associated with such high powers. Typically, RF power is coupled into a high-power RF cavity via a waveguide and a hermetic RF window. This approach, while viable at high power LINAC applications, requires additional hardware, which increases the cost, size and complexity of compact high power RF LINAC systems.
An alternative approach to the one described above is to couple RF power directly into the RF cavity via an RF amplifier assembly mounted on, and with an output stage coupled directly to, the RF cavity. This approach is described in Swenson, U.S. Pat. No. 5,084,682. However, the inclusion of the entire vacuum tube (and its associated tuning elements) within the vacuum envelope has led to an inability to operate at high powers due to processes such as multipactoring. For this reason, as much as possible of the RF and biasing circuitry needs to be at atmospheric pressure. In addition to this constraint, problems arise in the structure described in Swenson due to high powers dissipated both in the antenna and in the anode of the vacuum tube if these structures are not actively cooled. Swenson's approach to mounting the RF amplifier in a high power RF LINAC is further complicated by a vacuum tube anode commonly being held at high voltage, which necessitates the careful selection of a coolant.